The present invention relates to fasteners such as nuts for threaded bolts and the like, which are designed to be resistant to tampering thereby offering security against unwanted removal.
It is common in many applications to secure objects using threaded fasteners such as a nut and bolt combination where both the nut and bolt have cooperating threads. The nut is secured onto the bolt by the use of a suitable tool such as a wrench. The use of a nut and bolt combination provides a very secure attachment of objects, however, since the same commonly available tool is typically used to remove the nut, it is not possible to prevent unwanted removal. For this reason, various devices known as tamper-proof fasteners have been suggested which offer an element of security against unwanted removal. A key feature commonly incorporated into the design of such tamper-proof fasteners is a smooth outer surface, which makes it difficult to grasp with commonly available tools such as wrenches. A further key feature of tamper-proof fasteners is to design such fasteners to require a special key for tightening and releasing the fastener. Examples of such tamper resistant fasteners are given in the applicant""s previous U.S. Pat. No. 5,863,166 and other U.S. patents including U.S. Pat. Nos. 5,199,838, 4,674,306, 4,302,137, 4,125,051, 4,018,011, and UK patent application number 2,095,356.
Applicant""s previous U.S. Pat. No. 5,863,166 describes a device for attaching bicycle components to the bicycle frame. The device comprises a threaded bolt with a corresponding nut having a rotatable casing surrounding the outer surface and a key for engaging the nut. The key and the end face of the nut have at least one spigot and at least one hole such that the spigots of each fit in the holes of the other.
U.S. Pat. No. 5,199,838 teaches a nut for a threaded bolt, which has a spherical top surface and cylindrical sides. The nut is provided with a plurality of apertures into which fit complementary pins on a specially designed key. In this manner, only the uniquely complementary key can be used to turn the nut. Although such design makes removal difficult, it is still possible to rotate the nut using other tools such as needle nose pliers etc.
U.S. Pat. No. 4,674,306 describes a vehicle and assembly locking and wrenching apparatus where the vehicle lock is provided with the key for the tamperproof fastener. The tamperproof fastener is provided by a nut having an incised front face and arcuate sides parallel to the face surrounded by a cylindrical annular ring to prevent the gripping and turning of the nut.
U.S. Pat. No. 4,302,137 teaches a nut provided with a plurality of apertures. As before, a specially designed key with complementary pins is used to turn the nut. As a further security measure, this device is provided with a rotating collar, which makes it difficult to grasp the nut with common tools. A design of this type is complicated thereby adding to the manufacturing cost. Further, it is prone to the same type of tampering as in the ""838 patent discussed above.
U.S. Pat. No. 4,125,051 teaches a tamperproof fastener including a threaded shank and a head, which has a smooth circular side surface and a flat top surface. The top surface of the head is provided with a plurality of radially extending flanges, which cooperate with complementary flanges on a specific key. In one embodiment, the ""051 device allows for a removable fastener but it is indicated that reverse rotation of the fastener is not as efficient as the engagement direction. Thus, if the fastener is engaged too tightly, removal may not be possible.
U.S. Pat. No. 4,018,111 teaches a fastener having an axially extending threaded shank and a head having a smooth circular sidewall. The head also has a flat top surface with an offset aperture for receiving a complementary pin on a special key. The fastener is associated with a circular washer having a smooth frustoconical sidewall, which extends over the head thereby forming a smooth outer surface when the fastener is secured. This type of arrangement prevents tampering with a wrench, screwdriver or other such tool.
UK Patent Application teaches an antitheft device having a bolt with a cylindrical head surrounded by a rotatable cylindrical shell. The bolt head is provided with holes in a given pattern and the complementary key has extending pins in the same pattern as the holes and a hexagonal head capable of being engaged by a wrench. Once again, this design is expensive to manufacture
While a number of tamperproof fasteners have been developed, there still remains a need for a tamper resistant fastener or nut, which overcomes the deficiencies of the known devices.
In a preferred embodiment, the present invention provides a tamper resistant fastener which can only be rotated with a specific key. The fastener includes a head for rotation about a vertical axis. The head is provided with the least two cutouts in the head of the fastener spaced away from the vertical axis, the cutouts having a discontinuous sidewall. The side wall of each of the cutouts has a first generally vertical engaging surface for engagement by a mating member on a specific key for clockwise rotation of the head about the vertical axis and a second generally vertical engaging surface for engagement by the mating member on the specific key for counter clockwise rotation of the head about vertical axis. The side wall of the cutout is interrupted adjacent the second engaging surface to prevent engagement by a tool other than the specific key and thereby prevent rotation of the fastener in a counter clockwise direction by a tool other than the specific key.
In an aspect of the invention, the head has an upper convex surface extending to a bottom surface.
In another aspect of the invention, the head is provided with three cutouts spaced around the periphery of the head.
In yet another aspect of the invention, the cutouts have a generally circular mating member receiving recess, the discontinuous side wall being formed by removing material form the head adjacent the second generally vertical surface. The mating members of the key are generally circular projections extending downwardly from the lower surface.
Yet another aspect of the invention provides for a tamper resistant fastening system comprising a tamper proof fastener for attaching objects, the tamper proof fastener having a head for rotation about a vertical axis, and a specific key for rotating the fastener. The head of the fastener is provided with at least two cutouts spaced away from the vertical axis, the cutouts having a discontinuous sidewall. The side wall of each of the cutouts has a first generally vertical engaging surface for engagement by a mating member on the specific key for clockwise rotation of the head about the vertical axis and a second generally vertical engaging surface for engagement by the mating member on the specific key for counter clockwise rotation of the head about vertical axis. The side wall of the cutout is interrupted adjacent the second engaging surface to prevent engagement by a tool other than the specific key and thereby prevent rotation of the fastener in a counter clockwise direction by a tool other than the specific key.